1. Field of the Invention
The present invention is directed generally to magnetic heads for data storage applications, and more particularly, to thin film magnetic heads having flux-guided-MR or yoke-MR read sensors and methods for forming such heads.
2. Description of the Related Art
MR (magnetoresistive) sensors made by thin-film processes have been widely used in recent years for magnetic storage applications, particularly in disk drives. One of the properties of these sensors is that they vary in electrical resistance when placed in magnetic fields, such as the magnetic domains that represent recorded information on magnetic storage media. In disk drive applications, an MR read head having an MR sensor therein is carried on a slider mounted on a suspension. The suspension, in turn, is mounted to an actuator. The suspension mechanically biases the slider toward a surface of the disk. When the disk rotates, the loading is counterbalanced by a cushion of air (an "air bearing") generated by the rotating disk. The actuator moves the head to selected information tracks on the rotating magnetic disk. The resistance of the MR sensor changes in proportion to the change in magnetic field intensity caused by rotation of the disk. When a sense current is conducted through the MR sensor, the current changes in proportion to the change in resistance. Changes in the sense current are processed by a processor to produce playback signals corresponding to the information stored on the magnetic disk.
An MR sensor includes a stripe of MR material sandwiched between a pair of very thin insulative gap layers which are, in turn, sandwiched between a pair of magnetically conductive shield layers. Magnetic flux reaching the MR sensor extends through the gap layers to the shield layers. The MR sensor has front and back edges extending parallel to air bearing surface (ABS) of the slider on which it is carried. The MR sensor further includes a pair of side edges extending perpendicular to the ABS. The magnitude of magnetic flux that reaches the MR sensor is at a maximum at the front edge thereof. This magnitude decays along the MR stripe height and into the shields with a characteristic decay length. A boundary condition requires the flux magnitude to be zero at the back edge. When the stripe height of the MR sensor from the front to the back edge is less than the decay length, the flux loss along the height of the MR sensor is linear.
The MR sensor, which has a layer of magnetic material, is typically stabilized by a pair of hard bias (or exchange bias) layers adjacent its side edges. The hard bias layers longitudinally bias the MR sensor parallel to the ABS and stabilize the MR sensor from a multi-magnetic domain state to a single magnetic domain state. Accordingly, upon the termination of flux incursions into the MR sensor, the sensor always returns to a stabilized single magnetic domain state. Without longitudinal biasing, the domain walls of multimagnetic domains shift positions within the sensor, causing Barkhausen noise. This decreases the signal to noise ratio.
In some MR heads, the front edge of the MR sensor is exposed at the ABS and interfaces with the air bearing. To increase the amount of magnetic flux sensed by the MR sensor, and the intensity of the readback signal, a flux guide is connected to the back edge of the MR sensor and extends away from the ABS. In other MR heads, the MR sensor is recessed from the ABS. A second flux guide is connected to the front edge of the MR sensor and extends to the ABS. The second flux guide carries flux from the ABS to the MR sensor.
Recessed MR sensors are being pursued as attractive alternatives to MR sensors positioned at the ABS. Although the signal strength is reduced when the MR sensor is removed from the ABS, significant advantages achieved in the mechanical, electrical and chemical properties of the head may provide an acceptable tradeoff in many applications. From a mechanical standpoint, decreasing flying heights become possible and thermal asperity problems are minimized because there is less likelihood that the MR sensor will be damaged should the head contact the disk surface. From an electrical standpoint, the front flux guide insulates the current-carrying MR sensor and reduces the possibility of electrostatic discharge between the sensor and the disk. From a chemical standpoint, removal of the MR sensor from the disk means there is less chance that corrosion of the sensor will damage the disk. It therefore becomes possible to use less corrosion resistant materials for the MR sensor, which may provide performance benefits and reduce manufacturing costs.
For high density, post 1 Gbit/sq. in. trackwidths, it is generally accepted that the flux guides in recessed and nonrecessed heads need to be stabilized with hard bias (or exchange bias) by pinning the two sides of the flux guides with magnetically matched plugs in a fashion similar to the hard bias (or exchange bias) layers adjacent the MR sensor. Proper magnetic stabilization of MR heads thus requires the use of proper bias matching to both the MR sensor and the flux guides. The flux guide to sensor thickness ratio can be as large as twenty (20), particularly in yoke-MR heads, where one of the shields is used as a flux guide. As such, two separate depositions of hard bias material at different thicknesses are typically required.
One of the challenges of building a flux-guided-MR or yoke-MR sensor is the alignment of various edges with respect to each other to optimize flux guiding efficiency and proper bias matching. FIGS. 1A-1C show a sequence for building an MR sensor, which in the example is shown as having a sensor element that is recessed, but which could also have a nonrecessed sensor for purposes of the present discussion. Both the sensor element, which is made from a magnetoresistive material "MR," and the flux guide elements, which are made from a flux guide material "FG," are stabilized with separate longitudinal bias regions made from hard bias materials "HB1" and "HB2," respectively. Exchange bias could also be used. The structure shown in FIGS. 1A-1C is common for both yoke-MR heads, where one of the shields provides the flux guides and the other shield provides a return pole above the illustrated structure (i.e. in the z-axis direction), and flux-guided MR heads, where separate flux guides are employed and the illustrated structure is embedded between two shields. In FIG. 1A, an MR sensor layer 2 is deposited on a substrate 3, followed by two HB1 hard bias layers 4 and 6 formed along the sides of the MR sensor layer 2. In FIG. 1B, a pair of FG flux guides 8 and 10 are deposited on the substrate adjacent the front and rear edges of the MR sensor 2 and the hard bias layers 4 and 6. In FIG. 1C, two HB2 hard bias layers 12 and 14 are formed on the flux guide 8 and two HB2 hard bias layers 16 and 18 are formed on the flux guide 10. The critical point in the process is achieving y-direction alignment of the flux guide hard bias material layers 12-18 relative to the flux guides 8 and 10 on one hand, and the MR sensor 2 on the other. Misalignment of the HB2 hard bias layers causes either an overlap of the hard bias material into the MR sensor area or an absence of hard bias material in the flux guide area, resulting in the overbias of the sensor or an underbias of the flux guide material. The misalignment is illustrated by the structure shown in FIG. 1D.
The prior art method of making junctions between head components thus does not precisely position a hard bias layer at the edge portions of the flux guide and the MR sensor. The reason for this is because the prior art employs two resist masking steps. Under the best of conditions the alignment of a critical edge of the resist mask from a benchmark on a wafer is within +/-0.1 um. Another problem arises from unpredictable shrinkage of the resist. The location of the critical edge of the resist due to shrinkage varies +/-0.1 um. Even when windage is employed to attenuate the shrinkage problem, shrinkage is still variable from wafer to wafer. For any wafer containing multiple MR heads with flux guides and hard bias layers constructed according to the prior art, the yield will thus be unacceptably low because of the variability in hard bias layer alignment.
Accordingly, one cannot rely on photolithographic alignment for hard bias positioning in MR heads. An improved process is needed to ensure proper hard bias alignment and thereby improve the magnetic stabilization of the sensor and flux guides. What is required is an efficient method for closely controlling the deposition of the hard bias layers with consistent accuracy in order to improve manufacturing yields.